This application relates to the field of self contained, portable machines and test equipment for optical detectors, such as CCD and CMOS imaging devices, and more particularly to an automated, compact, efficient illumination system for use with a small integrating sphere which produces light having high spatial uniformity, high resolution MTF target to be used for characterizing the resolution of pixelized devices under test.
One of the fastest growing segments of the electro-optic art involves the use of photo electric detector arrays used in cameras and detectors for consumers, machine imaging and inspection imaging. The advancement in this area has been so extensive and so rapid at the technically advanced side of the market that the technology has enabled individually owned electronic cameras to begin to supplant cameras which use film and chemical development. The more technical side of the electronic imaging industry continues to advance and demands ever increasing sensitivity to produce a product of ever increasing quality. Increased affordability is had through mass production and the lowering of production costs while keeping the product quality high. High product quality is absolutely dependent upon high level testing.
To consider a simple electronic camera as an example, the main component is a two dimensional electronic array, typically a silicon-based device having thousands of pixels of a size less than 20 micrometers each. In more advanced applications, the array may be a three dimensional electronic array having an ability to make further measurements on light waves which may have penetrated the surface and which may have interfered with each other, for example. The remaining parts of the camera are far less critical and include a lens, a focusing system for physically moving the lens, and computer memory storage. The quality and suitability of the two dimensional detector array will determine whether the camera will function properly. As it is the most expensive and critical component in the camera, if it is defective, the camera as a whole is virtually worthless. Further, if the optical chip can be identified as rejected or accepted at an early stage of manufacture, before further assembly costs, significant efficiency and cost savings can be attained.
The critical need is therefore to properly test two dimensional arrays with as much speed and accuracy as possible to eliminate the defective components very early in the manufacturing process, at each stage before additional value can be added. Quality control is of paramount importance in the products which use two dimensional detector arrays, but even the tightest production and quality testing program cannot achieve its goals without the very most efficient test equipment. This problem is significant for low end products like ordinary digital cameras, but it is acute for high end and specialty two dimensional array products. Commercially available test illuminators can produce uniform illumination so long as the integrating sphere is large, significantly larger than the area occupied by the arrays to be tested.
Integrating spheres as commonly used have as their purpose the production of a uniform light source. The larger the integrating sphere, the more uniform light source produced. However, larger integrating spheres which overlie two dimensional arrays are more bulky to operate. Many of the smaller integrating spheres often fail to produce enough uniformity in illumination and do not provide uniform coverage over areas larger than about 24 square millimeters. Currently available illumination test equipment fails to give the greatest efficiency both because of failure in spatial illumination and uniformity and because of losses in illumination intensity resulting in inefficiency.
Testing is critical for several reasons. Any further work done on the two dimensional array if it is defective represents both lost time and lost material. Further, the average reasonable number of tests available for a given array are likely to be large in number and to additionally be dependent upon a specific set of testing criteria for the composition of the array, the intended use environment for the array or both. As such, doing really excellent testing translates into a really burdensome time and effort cost.
What is therefore needed is a test system which can perform intensive testing of a wide variety of two dimensional arrays, to simulate a further wide variety of operational environments to insure that arrays chosen for further processing are as close to perfect as the intended device requires.
The theory behind the operation and use of an integrator begins with the fact that proper evaluation of the functional performance of large two dimensional detector arrays for camera vision requires spatially uniform levels of illumination. Further, filtering and test patterns may be applied to test two dimensional arrays in an attempt to find even the smallest defect in the array. Commercially available simple test illuminators are low in efficiency, large, and bulky, and require an entry setup and calibration for each array tested. Existing illuminators have achieved spatial uniformity approaching a one percent variance taken over a rather small illuminating area. This value is unacceptable where high quality and very tight production control is essential. Without more, the use of a spherical integrator to attempt to statistically randomize the distribution of illumination is simply insufficient. Such conventional reflecting spheres attempt to provide a uniform nearly ideal distribution of light, known as Lambertian distribution, where the reflected intensity is substantially independent of the angle of incidence. However, commercially available test illuminators are low in efficiency, large and bulky and do not provide uniform illumination coverage over the minimum required coverage area. The output or reflective efficiency is a function of the overall area occupied by the radiating lamp, and may be difficult to control. Given this low level of efficiency, attempted compensation requires the use of a very high wattage lamp to power the illumination test system. A heating problem is thus created since about 80% of the energy going into the bulb is given off as waste heat which needs to be dissipated. Heat dissipation by providing openings in the sphere decrease would decrease its efficiency even further. A pure air ventilation system to compensate for the heat load would probably require refrigeration in order to work optimally. Resulting temperature changes from heating will introduce error into the two dimensional array measurement.
Where the wall is depended upon for providing the spatial uniformity, the disadvantages are cost, large size and bulk and especially the waste heat energy which is not only a problem in itself, but as a source of error as stray light which can in an unwanted manner heat the two dimensional array.
It is desirable to provide a relatively smaller beam cross section so that the homogeneity can be controlled. In the needed integrating sphere system it would be necessary to provide additional optics to accommodate economical filter sizes, and to provide for automated testing. A structure is needed which is portable, efficient, stable, compact and which can in an automated way test thousands of arrays in the minimum time.
A structure and system is provided for both avoiding the limitations on the currently available test devices and providing a source of uniform illumination that is compact, efficient and portable, and employing it in a wide variety of test set ups. The advantage to this structure and process, and overall approach is that a relatively cheap, fast, and compact illuminator can be manufactured and which will be so automated that it can be self tested through a high number of data points for a given array, as well as characterize a high number of arrays of one type and then be altered to test a completely different array in a matter of minutes. A light source uses a sphere to create a stream of uniform light through an exit aperture or exit port. A pair of motor driven filter wheels are mounted in front of the exit aperture or exit port along with a motor driven target slide. Control electronics are housed within the same housing as the sphere and provide rapid control for the filter wheels and exit aperture.
The advantage of the concept is both forward and reverse oriented. A known test array having known characteristics can be used to calibrate the expected results in order to get a real time indication of the performance and state of the illuminator. Once performance characteristics have been established, a level of performance may be specified before introduction of the two-dimensional arrays to be tested. The mass testing of arrays may then begin. After a reasonable period of time, the test array can be reintroduced to insure that no defects in the illuminator have developed from either changes in the illumination source, heat, or variations in power, and the like. As a result, consistency is assured.
During the test, light from the elliptical light source/concentrator is directed through a field homogenizer and shutter, and then through controlled position spectral filter and attenuation wheels, such individual filter and attenuation materials may be commonly commercially available. The spectral filter and attenuation wheels are driven by a filter wheel/shutter drive controller. Light directed through the field homogenizer and shutter, and spectral filter and attenuation wheel is thus further smoothed of its spatial unevenness, before being directed through a lens transfer system to then produce uniform pupil irradiance.
The light source is preferably a high temperature tungsten halogen lamp or quartz halogen. The lamp can be chosen from commercially available lamps and is preferably positioned at one focus of a sphere.